Effects of nitric oxide on horizontal cells in the rabbit retina

Layers of inhibitory networks shape receptive field properties of AII amacrine cells

In the retina, rod and cone pathways mediate visual signals over a billion-fold range in luminance. AII ("A-two") amacrine cells (ACs) receive signals from both pathways via different bipolar cells, enabling AIIs to operate at night and during the day. Previous work has examined luminance-dependent changes in AII gap junction connectivity, but less is known about how surrounding circuitry shapes AII receptive fields across light levels. Here, we report that moderate contrast stimuli elicit surround inhibition in AIIs under all but the dimmest visual conditions, due to actions of horizontal cells and at least two ACs that inhibit presynaptic bipolar cells. Under photopic (daylight) conditions, surround inhibition transforms AII response kinetics, which are inherited by downstream ganglion cells. Ablating neuronal nitric oxide synthase type-1 (nNOS-1) ACs removes AII surround inhibition under mesopic (dusk/dawn), but not photopic, conditions. Our findings demonstrate how multiple layers of neural circuitry interact to encode signals across a wide physiological range.

Extrinsic and Intrinsic Factors Determine Expression Levels of Gap Junction-Forming Connexins in the Mammalian Retina

Gap junctions (GJs) are not static bridges; instead, GJs as well as the molecular building block connexin (Cx) proteins undergo major expression changes in the degenerating retinal tissue. Various progressive diseases, including retinitis pigmentosa, glaucoma, age-related retinal degeneration, etc., affect neurons of the retina and thus their neuronal connections endure irreversible changes as well. Although Cx expression changes might be the hallmarks of tissue deterioration, GJs are not static bridges and as such they undergo adaptive changes even in healthy tissue to respond to the ever-changing environment. It is, therefore, imperative to determine these latter adaptive changes in GJ functionality as well as in their morphology and Cx makeup to identify and distinguish them from alterations following tissue deterioration. In this review, we summarize GJ alterations that take place in healthy retinal tissue and occur on three different time scales: throughout the entire lifespan, during daily changes and as a result of quick changes of light adaptation.

Light and myopia: from epidemiological studies to neurobiological mechanisms

Myopia is far beyond its inconvenience and represents a true, highly prevalent, sight-threatening ocular condition, especially in Asia. Without adequate interventions, the current epidemic of myopia is projected to affect 50% of the world population by 2050, becoming the leading cause of irreversible blindness. Although blurred vision, the predominant symptom of myopia, can be improved by contact lenses, glasses or refractive surgery, corrected myopia, particularly high myopia, still carries the risk of secondary blinding complications such as glaucoma, myopic maculopathy and retinal detachment, prompting the need for prevention. Epidemiological studies have reported an association between outdoor time and myopia prevention in children. The protective effect of time spent outdoors could be due to the unique characteristics (intensity, spectral distribution, temporal pattern, etc.) of sunlight that are lacking in artificial lighting. Concomitantly, studies in animal models have highlighted the efficacy of light and its components in delaying or even stopping the development of myopia and endeavoured to elucidate possible mechanisms involved in this process. In this narrative review, we (1) summarize the current knowledge concerning light modulation of ocular growth and refractive error development based on studies in human and animal models, (2) summarize potential neurobiological mechanisms involved in the effects of light on ocular growth and emmetropization and (3) highlight a potential pathway for the translational development of noninvasive light-therapy strategies for myopia prevention in children.

An Alternative Splice Variant of Zebrafish Cx52.6 is Expressed in Retinal Horizontal Cells

Retinal horizontal cells (HCs) are inhibitory neurons, which modulate the transmission of light-elicited signals from photoreceptors to bipolar cells in the outer retina. HCs of the same physiological type are extensively coupled via gap junctions. In the zebrafish retina, the population of HCs comprises up to four morphologically distinct subtypes. Four different connexins (Cx52.6, Cx52.7, Cx52.9 and Cx55.5) were detected in these cells with overlapping expression patterns. In this study, we show that Cx52.6 is alternatively spliced in the retina, resulting in an additional isoform, designated as Cx53.4, which differs from the originally described Cx52.6 only by the final C-terminal peptide (12 vs. 4 aa). Further protein sequence alignments revealed that Cx53.4 represents the counterpart of alternatively spliced mouse Cx57 and human Cx62. RT-PCR analyses of mRNA expression in different adult zebrafish tissues showed that Cx53.4 is expressed exclusively in the retina. The localization of Cx53.4 protein within the retina was analyzed using a specific antibody. Immunofluorescence analyses demonstrated that the expression of Cx53.4 is restricted to HCs of all four subtypes. Further, immunoelectron microscopy confirmed the presence of Cx53.4 in gap junctions between HC dendrites and between their axon terminals.

Müller stem cell dependent retinal regeneration

Müller Stem cells to treat ocular diseases has triggered enthusiasm across all medical and scientific communities. Recent development in the field of stem cells has widened the prospects of applying cell based therapies to regenerate ocular tissues that have been irreversibly damaged by disease or injury. Ocular tissues such as the lens and the retina are now known to possess cell having remarkable regenerative abilities. Recent studies have shown that the Müller glia, a cell found in all vertebrate retinas, is the primary source of new neurons, and therefore are considered as the cellular basis for retinal regeneration in mammalian retinas. Here, we review the current status of retinal regeneration of the human eye by Müller stem cells. This review elucidates the current status of retinal regeneration by Müller stem cells, along with major retinal degenerative diseases where these stem cells play regenerative role in retinal repair and replacement.

Complexity of gap junctions between horizontal cells of the carp retina

In the vertebrate retina, horizontal cells (HCs) reveal homologous coupling by gap junctions (gj), which are thought to consist of different connexins (Cx). However, recent studies in mouse, rabbit and zebrafish retina indicate that individual HCs express more than one connexin. To provide further insights into the composition of gj connecting HCs and to determine whether HCs express multiple connexins, we examined the molecular identity and distribution of gj between HCs of the carp retina. We have cloned four carp connexins designated Cx49.5, Cx55.5, Cx52.6 and Cx53.8 with a close relationship to connexins previously reported in HCs of mouse, rabbit and zebrafish, respectively. Using in situ hybridization, Cx49.5 expression was detected in different subpopulations of retinal neurons including HCs, whereas the Cx52.6 transcript was localized exclusively in HCs. Using specific antibodies, Cx55.5 and Cx53.8 were detected on dendrites of all four HC subtypes and axon terminals. Immunoelectron microscopy confirmed the presence of Cx55.5 and Cx53.8 in gap junctions between these processes and Cx55.5 was additionally observed in HC dendrites invaginating cone pedicles, suggesting its participation in the modulation of photoreceptor output in the carp retina. Furthermore, using single-cell RT-PCR, all four connexins were detected in different subtypes of HCs, suggesting overlapping expression patterns. Thus, the composition of gj mediating homologous coupling between subtypes of carp HCs appears to be more complex than expected. Moreover, BLAST searches of the preliminary carp genome, using novel sequences as query, suggest that most of the analyzed connexin genes are duplicated in carp.

Regulation of gap junction channels and hemichannels by phosphorylation and redox changes: a revision

Post-translational modifications of connexins play an important role in the regulation of gap junction and hemichannel permeability. The prerequisite for the formation of functional gap junction channels is the assembly of connexin proteins into hemichannels and their insertion into the membrane. Hemichannels can affect cellular processes by enabling the passage of signaling molecules between the intracellular and extracellular space. For the intercellular communication hemichannels from one cell have to dock to its counterparts on the opposing membrane of an adjacent cell to allow the transmission of signals via gap junctions from one cell to the other. The controlled opening of hemichannels and gating properties of complete gap junctions can be regulated via post-translational modifications of connexins. Not only channel gating, but also connexin trafficking and assembly into hemichannels can be affected by post-translational changes. Recent investigations have shown that connexins can be modified by phosphorylation/dephosphorylation, redox-related changes including effects of nitric oxide (NO), hydrogen sulfide (H2S) or carbon monoxide (CO), acetylation, methylation or ubiquitination. Most of the connexin isoforms are known to be phosphorylated, e.g. Cx43, one of the most studied connexin at all, has 21 reported phosphorylation sites. In this review, we provide an overview about the current knowledge and relevant research of responsible kinases, connexin phosphorylation sites and reported effects on gap junction and hemichannel regulation. Regarding the effects of oxidants we discuss the role of NO in different cell types and tissues and recent studies about modifications of connexins by CO and H2S.

Schizophrenia and the eye

Although visual processing impairments are common in schizophrenia, it is not clear to what extent these originate in the eye vs. the brain. This review highlights potential contributions, from the retina and other structures of the eye, to visual processing impairments in schizophrenia and high-risk states. A second goal is to evaluate the status of retinal abnormalities as biomarkers for schizophrenia. The review was motivated by known retinal changes in other disorders (e.g., Parkinson’s disease, multiple sclerosis), and their relationships to perceptual and cognitive impairments, and disease progression therein. The evidence reviewed suggests two major conclusions. One is that there are multiple structural and functional disturbances of the eye in schizophrenia, all of which could be factors in the visual disturbances of patients. These include retinal venule widening, retinal nerve fiber layer thinning, dopaminergic abnormalities, abnormal ouput of retinal cells as measured by electroretinography (ERG), maculopathies and retinopathies, cataracts, poor acuity, and strabismus. Some of these are likely to be illness-related, whereas others may be due to medication or comorbid conditions. The second conclusion is that certain retinal findings can serve as biomarkers of neural pathology, and disease progression, in schizophrenia. The strongest evidence for this to date involves findings of widened retinal venules, thinning of the retinal nerve fiber layer, and abnormal ERG amplitudes. These data suggest that a greater understanding of the contribution of retinal and other ocular pathology to the visual and cognitive disturbances of schizophrenia is warranted, and that retinal changes have untapped clinical utility.

Functional regulation of neuronal nitric oxide synthase expression and activity in the rat retina

In the nervous system within physiological conditions, nitric oxide (NO) production depends on the activity of nitric oxide synthases (NOSs), and particularly on the expression of the neuronal isoform (nNOS). In the sensory systems, the role of NO is poorly understood. In this study, we identified nNOS-positive cells in the inner nuclear layer (INL) of the rat retina, with distinct characteristics such as somata size, immunolabeling level and location. Employing mathematical cluster analysis, we determined that nNOS amacrine cells are formed by two distinct populations. We next investigated the molecular identity of these cells, which did not show colocalization with calbindin (CB), choline acetyltransferase (ChAT), parvalbumin (PV) or protein kinase C (PKC), and only partial colocalization with calretinin (CR), revealing the accumulation of nNOS in specific amacrine cell populations. To access the functional, circuitry-related roles of these cells, we performed experiments after adaptation to different ambient light conditions. After 24h of dark-adaptation, we detected a subtle, yet statistically significant decrease in nNOS transcript levels, which returned to steady-state levels after 24h of normal light-dark cycle, revealing that nNOS expression is governed by ambient light conditions. Employing electron paramagnetic resonance (EPR), we demonstrated that dark-adaptation decreases NO production in the retina. Furthermore, nNOS accumulation changed in the dark-adapted retinas, with a general reduction in the inner plexiform layer. Finally, computational analysis based on clustering techniques revealed that dark-adaptation differently affected both types of nNOS-positive amacrine cells. Taken together, our data disclosed functional regulation of nNOS expression and activity, disclosing new circuitry-related roles of nNOS-positive cells. More importantly, this study indicated unsuspected roles for NO in the sensory systems, particularly related to adaptation to ambient demands.

Variety of alternative stable phase-locking in networks of electrically coupled relaxation oscillators

We studied the dynamics of a large-scale model network comprised of oscillating electrically coupled neurons. Cells are modeled as relaxation oscillators with short duty cycle, so they can be considered either as models of pacemaker cells, spiking cells with fast regenerative and slow recovery variables or firing rate models of excitatory cells with synaptic depression or cellular adaptation.

It was already shown that electrically coupled relaxation oscillators exhibit not only synchrony but also anti-phase behavior if electrical coupling is weak. We show that a much wider spectrum of spatiotemporal patterns of activity can emerge in a network of electrically coupled cells as a result of switching from synchrony, produced by short external signals of different spatial profiles. The variety of patterns increases with decreasing rate of neuronal firing (or duty cycle) and with decreasing strength of electrical coupling. We study also the effect of network topology - from all-to-all – to pure ring connectivity, where only the closest neighbors are coupled.

We show that the ring topology promotes anti-phase behavior as compared to all-to-all coupling. It also gives rise to a hierarchical organization of activity: during each of the main phases of a given pattern cells fire in a particular sequence determined by the local connectivity. We have analyzed the behavior of the network using geometric phase plane methods and we give heuristic explanations of our findings.

Our results show that complex spatiotemporal activity patterns can emerge due to the action of stochastic or sensory stimuli in neural networks without chemical synapses, where each cell is equally coupled to others via gap junctions. This suggests that in developing nervous systems where only electrical coupling is present such a mechanism can lead to the establishment of proto-networks generating premature multiphase oscillations whereas the subsequent emergence of chemical synapses would later stabilize generated patterns.

Nitric oxide as a regulatory molecule in the processing of the visual stimulus

Nitric oxide (NO) is a highly reactive gas with considerable diffusion power that is produced pre- and post synaptically in the central nervous system (CNS). In the visual system, it is involved in the processing of the visual information from the retina to superior visual centers. In this review we discuss the main mechanisms through which nitric oxide acts, in physiological levels, on the retina, lateral geniculate nucleus (LGN) and primary visual cortex. In the retina, the cGMP-dependent nitric oxide activity initially amplifies the signal, subsequently increasing the inhibitory activity, suggesting that the signal is "filtered". In the thalamus, on dLGN, neuronal activity is amplified by NO derived from brainstem cholinergic cells, in a cGMP-independent mechanism; the result is the amplification of the signal arriving from retina. Finally, on the visual cortex (V1), NO acts through changes on the cGMP levels, increasing signal detection. These observations suggest that NO works like a filter, modulating the signal along the visual pathways.

Nitric oxide production and the expression of two nitric oxide synthases in the avian retina

Nitric oxide (NO) is known to exert multiple effects on the function of many retinal neurons and their synapses. Therefore, it is equally important to understand the potential sources of NO within the retina. To explore this, we employ a combination of 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM) based NO detection and immunohistochemistry for the NO synthetic enzymes, neuronal and endothelial nitric oxide synthase (nNOS and eNOS). We find DAF signals in photoreceptors, horizontal cells, amacrine cells, efferent synapses, Müller cells, and cells in the ganglion cell layer (GCL). nNOS immunoreactivity was consistent with the DAF signal with the exception that horizontal cells and Müller cells were not clearly labeled. eNOS-like immunoreactivity (eNOS-LI) was more widespread with photoreceptors, horizontal cells, occasional bipolar cells, amacrine cells, Müller cells, and cells in the GCL all showing labeling. Double labeling with antibodies raised against calretinin, syntaxin, and glutamine synthetase confirmed that horizontal cells, amacrine cells, and Müller cells (respectively) were expressing eNOS-LI. Although little or no nNOS labeling is observed in horizontal cells or Müller cells, the expression of eNOS-LI is consistent with the ability of these cells to produce NO. Together these results suggest that the capability to produce NO is widespread in the chicken retina. We propose that multiple forms of regulation for nNOS and eNOS play a role in the patterning of NO production in the chicken retina.

Histamine receptors of cones and horizontal cells in Old World monkey retinas

In primates the retina receives input from histaminergic neurons in the posterior hypothalamus that are active during the day. In order to understand how this input contributes to information processing in Old World monkey retinas, we have been localizing histamine receptors (HR) and studying the effects of histamine on the neurons that express them. Previously, we localized HR3 to the tips of ON bipolar cell dendrites and showed that histamine hyperpolarizes the cells via this receptor. We raised antisera against synthetic peptides corresponding to an extracellular domain of HR1 between the 4th and 5th transmembrane domains and to an intracellular domain near the carboxyl terminus of HR2. Using these, we localized HR1 to horizontal cells and a small number of amacrine cells and localized HR2 to puncta closely associated with synaptic ribbons inside cone pedicles. Consistent with this, HR1 mRNA was detected in horizontal cell perikarya and primary dendrites and HR2 mRNA was found in cone inner segments. We studied the effect of 5 μM exogenous histamine on primate cones in macaque retinal slices. Histamine reduced I(h) at moderately hyperpolarized potentials, but not the maximal current. This would be expected to increase the operating range of cones and conserve ATP in bright, ambient light. Thus, all three major targets of histamine are in the outer plexiform layer, but the retinopetal axons containing histamine terminate in the inner plexiform layer. Taken together, the findings in these three studies suggest that histamine acts primarily via volume transmission in primate retina.

Cut-loading: a useful tool for examining the extent of gap junction tracer coupling between retinal neurons

In addition to chemical synaptic transmission, neurons that are connected by gap junctions can also communicate rapidly via electrical synaptic transmission. Increasing evidence indicates that gap junctions not only permit electrical current flow and synchronous activity between interconnected or coupled cells, but that the strength or effectiveness of electrical communication between coupled cells can be modulated to a great extent^1,2. In addition, the large internal diameter (~1.2 nm) of many gap junction channels permits not only electric current flow, but also the diffusion of intracellular signaling molecules and small metabolites between interconnected cells, so that gap junctions may also mediate metabolic and chemical communication. The strength of gap junctional communication between neurons and its modulation by neurotransmitters and other factors can be studied by simultaneously electrically recording from coupled cells and by determining the extent of diffusion of tracer molecules, which are gap junction permeable, but not membrane permeable, following iontophoretic injection into single cells. However, these procedures can be extremely difficult to perform on neurons with small somata in intact neural tissue.

Numerous studies on electrical synapses and the modulation of electrical communication have been conducted in the vertebrate retina, since each of the five retinal neuron types is electrically connected by gap junctions^3,4. Increasing evidence has shown that the circadian (24-hour) clock in the retina and changes in light stimulation regulate gap junction coupling^3-8. For example, recent work has demonstrated that the retinal circadian clock decreases gap junction coupling between rod and cone photoreceptor cells during the day by increasing dopamine D2 receptor activation, and dramatically increases rod-cone coupling at night by reducing D2 receptor activation^7,8. However, not only are these studies extremely difficult to perform on neurons with small somata in intact neural retinal tissue, but it can be difficult to adequately control the illumination conditions during the electrophysiological study of single retinal neurons to avoid light-induced changes in gap junction conductance.

Here, we present a straightforward method of determining the extent of gap junction tracer coupling between retinal neurons under different illumination conditions and at different times of the day and night. This cut-loading technique is a modification of scrape loading^9-12, which is based on dye loading and diffusion through open gap junction channels. Scrape loading works well in cultured cells, but not in thick slices such as intact retinas. The cut-loading technique has been used to study photoreceptor coupling in intact fish and mammalian retinas^{7, 8,13}, and can be used to study coupling between other retinal neurons, as described here.

Nitric oxide signaling in the retina: what have we learned in two decades?

Two decades after its first detection in the retina, nitric oxide (NO) continues to puzzle visual neuroscientists. While its liberation by photoreceptors remains controversial, recent evidence supports three subtypes of amacrine cells as main sources of NO in the inner retina. NO synthesis was shown to depend on light stimulation, and mounting evidence suggests that NO is a regulator of visual adaptation at different signal processing levels. NO modulates light responses in all retinal neuron classes, and specific ion conductances are activated by NO in rods, cones, bipolar and ganglion cells. Light-dependent gap junction coupling in the inner and outer plexiform layers is also affected by NO. The vast majority of these effects were shown to be mediated by activation of the NO receptor soluble guanylate cyclase and resultant cGMP elevation. This review analyzes the current state of knowledge on physiological NO signaling in the retina.

Light- and dopamine-regulated receptive field plasticity in primate horizontal cells

Center-surround antagonistic receptive fields (CSARFs) are building blocks for spatial vision and contrast perception. Retinal horizontal cells (HCs) are the first lateral elements along the visual pathway, and are thought to contribute to receptive field surrounds of higher order neurons. Primate HC receptive fields have not been found to change with light, and dopaminergic modulation has not been investigated. Recording intracellularly from HCs in dark-adapted macaque retina, we found that H1-HCs had large receptive fields (λ = 1,158 ± 137 μm) that were reduced by background light (-45%), gap junction closure (-53%), and D1 dopamine receptor activation (-48%). Tracer coupling was modulated in a correlative manner, suggesting that coupling resistance plays a dominant role in receptive field formation under low light conditions. The D1 antagonist SCH23390 increased the size of receptive fields (+13%), suggesting tonic dopamine release in the dark. Because light elevates dopamine release in primate retina, our results support a dopaminergic role in post-receptoral light adaptation by decreasing HC receptive field diameters, which influences the center-surround receptive field organization of higher-order neurons and thereby spatial contrast sensitivity.

The light-induced reduction of horizontal cell receptive field size in the goldfish retina involves nitric oxide

Horizontal cells of the vertebrate retina have large receptive fields as a result of extensive gap junction coupling. Increased ambient illumination reduces horizontal cell receptive field size. Using the isolated goldfish retina, we have assessed the contribution of nitric oxide to the light-dependent reduction of horizontal cell receptive field size. Horizontal cell receptive field size was assessed by comparing the responses to centered spot and annulus stimuli and from the responses to translated slit stimuli. A period of steady illumination decreased the receptive field size of horizontal cells, as did treatment with the nitric oxide donor (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (100 μM). Blocking the endogenous production of nitric oxide with the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (1 mM), decreased the light-induced reduction of horizontal cell receptive field size. These findings suggest that nitric oxide is involved in light-induced reduction of horizontal cell receptive field size.

Role of connexins in microvascular dysfunction during inflammation

In arterioles, a locally initiated diameter change can propagate rapidly along the vessel length (arteriolar conducted response), thus contributing to arteriolar hemodynamic resistance. The response is underpinned by electrical coupling along the arteriolar endothelial layer. Connexins (Cx; constituents of gap junctions) are required for this coupling. This review addresses the effect of acute systemic inflammation (sepsis) on arteriolar conduction and interendothelial electrical coupling. Lipopolysaccharide (LPS; an initiating factor in sepsis) and polymicrobial sepsis (24 h model) attenuate conducted vasoconstriction in mice. In cultured microvascular endothelial cells harvested from rat and mouse skeletal muscle, LPS reduces both conducted hyperpolarization–depolarization along capillary-like structures and electrical coupling along confluent cell monolayers. LPS also tyrosine-phosphorylates Cx43 and serine-dephosphorylates Cx40. Since LPS-reduced coupling is Cx40- but not Cx43-dependent, only Cx40 dephosphorylation may be consequential. Nitric oxide (NO) overproduction is critical in advanced sepsis, since the removal of this overproduction prevents the attenuated conduction. Consistently, (i) exogenous NO in cultured cells reduces coupling in a Cx37-dependent manner, and (ii) the septic microvasculature in vivo shows no Cx40 phenotype. A complex role emerges for endothelial connexins in sepsis. Initially, LPS may reduce interendothelial coupling and arteriolar conduction by targeting Cx40, whereas NO overproduction in advanced sepsis reduces coupling and conduction by targeting Cx37 instead.

Stem cells therapy for retinal degeneration

Stem cell therapy is widely considered as a therapeutic approach for retinal degeneration. Retinal injury results in permanent visual disturbance or blindness. Repair of such damage by stem cells is one of the most feasible types of central nervous system repair. In this review, we consider how stem cells might be optimized for use as donor cells. We discuss the benefits of stem cells for transplantation in retinal degenerative disease. A wide range of stem cells from different sources is being investigated for the treatment of retinal degeneration. This study reviews the recent and old achievements about stem cells for retinal repair.

Light responses and morphology of bNOS-immunoreactive neurons in the mouse retina

Nitric oxide (NO), produced by NO synthase (NOS), modulates the function of all retinal neurons and ocular blood vessels and participates in the pathogenesis of ocular diseases. To further understand the regulation of ocular NO release, we systematically studied the morphology, topography, and light responses of NOS-containing amacrine cells (NOACs) in dark-adapted mouse retina. Immunohistological staining for neuronal NOS (bNOS), combined with retrograde labeling of ganglion cells (GCs) with Neurobiotin (NB, a gap junction permeable dye) and Lucifer yellow (LY, a less permeable dye), was used to identify NOACs. The light responses of ACs were recorded under whole-cell voltage clamp conditions and cell morphology was examined with a confocal microscope. We found that in dark-adapted conditions bNOS-immunoreactivity (IR) was present primarily in the inner nuclear layer and the ganglion cell layer. bNOS-IR somas were negative for LY, thus they were identified as ACs; nearly 6% of the cells were labeled by NB but not by LY, indicating that they were dye-coupled with GCs. Three morphological subtypes of NOACs (NI, NII, and displaced) were identified. The cell density, intercellular distance, and the distribution of NOACs were studied in whole retinas. Light evoked depolarizing highly sensitive ON-OFF responses in NI cells and less sensitive OFF responses in NII cells. Frequent (1-2 Hz) or abrupt change of light intensity evoked larger peak responses. The possibility for light to modify NO release from NOACs is discussed.

Connexin36 is required for gap junctional coupling of most ganglion cell subtypes in the mouse retina

Converging evidence indicates that electrical synaptic transmission via gap junctions plays a crucial role in signal processing in the retina. In particular, amacrine and ganglion cells express numerous gap junctions, resulting in extensive electrical networks in the proximal retina. Both connexin36 (Cx36) and connexin45 (Cx45) subunits are widely distributed in the inner plexiform layer (IPL) and therefore are likely contribute to gap junctions formed by a number of ganglion cell subtypes. In the present study, we used the gap junction-permeant tracer Neurobiotin to compare the coupling pattern of different ganglion cell subtypes in wild-type (WT) and Cx36 knockout (KO) mouse retinas. We found that homologous ganglion-to-ganglion cell coupling was lost for two subtypes after deletion of Cx36, whereas two other ganglion cell subtypes retained homologous coupling in the KO mouse. In contrast, deletion of Cx36 resulted in a partial or complete loss of ganglion-to-amacrine cell heterologous coupling in 9 of 10 ganglion cell populations studied. Overall, our results indicate that Cx36 is the predominant subunit of gap junctions in the proximal mouse retina, expressed by most ganglion cell subtypes, and thereby likely plays a major role in the concerted activity generated by electrical synapses.

Reduction of electrical coupling between microvascular endothelial cells by NO depends on connexin37

We have previously shown that increased nitric oxide (NO) production in sepsis impairs arteriolar-conducted vasoconstriction cGMP independently and that the gap junction protein connexin (Cx) 37 is required for this conducted response. In the present study, we hypothesized that NO impairs interendothelial electrical coupling in sepsis by targeting Cx37. We examined the effect of exogenous NO on coupling in monolayers of cultured microvascular endothelial cells derived from the hindlimb skeletal muscle of wild-type (WT), Cx37 null, Cx40 null, and Cx43(G60S) (nonfunctional mutant) mice. To assess coupling, we measured the spread of electrical current injected in the monolayer and calculated the monolayer intercellular resistance (inverse measure of coupling). The NO donor 2,2'-(hydroxynitrosohydrazino)bis-ethanamine (DETA) rapidly and reversibly reduced coupling in cells from WT mice, cGMP independently. NO scavenger HbO(2) did not affect baseline coupling, but it eliminated DETA-induced reduction in coupling. Reduced coupling in response to DETA was also seen in cells from Cx40 null and Cx43(G60S) mice, but not in cells from Cx37 null mice. DETA did not alter the expression of Cx37, Cx40, and Cx43 in WT cells analyzed by immunoblotting and immunofluorescence. Furthermore, neither the peroxynitrite scavenger 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato iron (III), superoxide scavenger Mn(III)tetrakis(4-benzoic acid)porphyrin chloride, nor preloading of WT cells with the antioxidant ascorbate affected this reduction. We conclude that NO-induced reduction of electrical coupling between microvascular endothelial cells depends on Cx37 and propose that NO in sepsis impairs arteriolar-conducted vasoconstriction by targeting Cx37 within the arteriolar wall.

The diverse functional roles and regulation of neuronal gap junctions in the retina

Electrical synaptic transmission through gap junctions underlies direct and rapid neuronal communication in the CNS. The diversity of functional roles that electrical synapses have is perhaps best exemplified in the vertebrate retina, in which gap junctions are formed by each of the five major neuron types. These junctions are dynamically regulated by ambient illumination and by circadian rhythms acting through light-activated neuromodulators such as dopamine and nitric oxide, which in turn activate intracellular signalling pathways in the retina.The networks formed by electrically coupled neurons are plastic and reconfigurable, and those in the retina are positioned to play key and diverse parts in the transmission and processing of visual information at every retinal level.

ZO-1 and the spatial organization of gap junctions and glutamate receptors in the outer plexiform layer of the mammalian retina

Information processing in the retina starts at the first synaptic layer, where photoreceptors and second-order neurons exhibit a complex architecture of glutamatergic and electrical synapses. To investigate the composition of this highly organized synaptic network, we determined the spatial relationship of zonula occludens-1 (ZO-1) with different connexins (Cx) and glutamate receptor (GluR) subunits in the outer plexiform layer (OPL) of rabbit, mouse, and monkey retinas. ZO-1 is well known as an intracellular component of tight and adherens junctions, but also interacts with various connexins at gap junctions. We found ZO-1 closely associated with Cx50 on dendrites of A-type horizontal cells in rabbit, and with Cx57 at dendro-dendritic gap junctions of mouse horizontal cells. The spatial arrangement of ZO-1 at the giant gap-junctional plaques in rabbit was particularly striking. ZO-1 formed a clear margin around the large Cx50 plaques instead of being colocalized with the connexin staining. Our finding suggests the involvement of ZO-1 in the composition of tight or adherens junctions around gap-junctional plaques instead of interacting with connexins directly. Furthermore, gap junctions were found to be clustered in close proximity to GluRs at the level of desmosome-like junctions, where horizontal cell dendrites converge before invaginating the cone pedicle. Based on this distinct spatial organization of gap junctions and GluRs, it is tempting to speculate that glutamate released from the photoreceptors may play a role in modulating the conductance of electrical synapses in the OPL.

Connexin57 is expressed in dendro-dendritic and axo-axonal gap junctions of mouse horizontal cells and its distribution is modulated by light

Mouse horizontal cells are coupled by gap junctions composed of connexin57. These gap junctions are regulated by ambient light via multiple neuromodulators including dopamine. In order to analyze the distribution and structure of horizontal cell gap junctions in the mouse retina, and examine the effects of light adaptation on gap junction density, we developed antibodies that detect mouse retinal connexin57. Using immunohistochemistry in retinal slices, flat-mounted retinas, and dissociated retinal cells, we showed that connexin57 is expressed in the dendrites and axon terminal processes of mouse horizontal cells. No staining was found in retinas of connexin57-deficient mice. Significantly more connexin57-positive puncta were found in the distal than in the proximal outer plexiform layer, indicating a higher level of expression in axon terminal processes than in the dendrites. We also examined the gap junctions using immunoelectron microscopy and showed that connexin57 does not form hemichannels in the horizontal cell dendritic tips. Light adaptation resulted in a significant increase in the number of connexin57-immunoreactive plaques in the outer plexiform layer, consistent with previously reported effects of light adaptation on connexin57 expression in the mouse retina. This study shows for the first time the detailed location of connexin57 expression within mouse horizontal cells, and provides the first ultrastructural data on mouse horizontal cell gap junctions.

Tracer coupling patterns of the ganglion cell subtypes in the mouse retina

It is now clear that electrical coupling via gap junctions is prevalent across the retina, expressed by each of the five main neuronal types. With the introduction of mutants in which selective gap junction connexins are deleted, the mouse has recently become an important model for studying the function of coupling between retinal neurons. In this study we examined the tracer-coupling pattern of ganglion cells by injecting them with the gap junction-permanent tracer Neurobiotin to provide, for the first time, a comprehensive survey of ganglion cell coupling in the wildtype mouse retina. Murine ganglion cells were differentiated into 22 morphologically distinct subtypes based on soma-dendritic parameters. Most (16/22) ganglion cell subtypes were tracer-coupled to neighboring ganglion and/or amacrine cells. The amacrine cells coupled to ganglion cells displayed either polyaxonal or wide-field morphologies with extensive arbors. We found that different subtypes of ganglion cells were never coupled to one another, indicating that they subserved independent electrical networks. Finally, we found that the tracer-coupling patterns of the 22 ganglion cell populations were largely stereotypic across the 71 retinas studied. Our results indicate that electrical coupling is extensive in the inner retina of the mouse, suggesting 0

The possible role of retinal dopaminergic system in visual performance

It is a well-known fact that the retina is one of the tissues in the body, which is richest in dopamine (DA), yet the role of this system in various visual functions remains unclear. We have identified 13 types of DA retinal pathologies, and 15 visual functions. The pathologies were arranged in this review on a net grid, where one axis was "age" (i.e., from infancy to old age) and the other axis the level of retinal DA (i.e., from DA deficiency to DA excess, from Parkinson disorder to Schizophrenia). The available data on visual dysfunction(s) is critically presented for each of the DA pathologies. Special effort was made to evaluate whether the site of DA malfunction in the different DA pathologies and visual function is at retinal level or in higher brain centers. The mapping of DA and visual pathologies demonstrate the pivot role of retinal DA in mediating visual functions and also indicate the "missing links" in our understanding of the mechanisms underlying these relationships.

Distribution of soluble guanylyl cyclase in rat retina

The nitric oxide (NO)-cGMP pathway is implicated in modulation of visual information processing in the retina. Despite numerous functional studies of this pathway, information about the retinal distribution of the major downstream effector of NO, soluble guanylyl cyclase (sGC), is very limited. In the present work, we have used immunohistochemistry and multiple labeling to determine the distribution of sGC in rat retina. sGC was present at high levels in inner retina but barely detectable in outer retina. Photoreceptors and horizontal cells, as well as Müller cells, were immunonegative, whereas retinal ganglion cells exhibited moderate staining for sGC. Strong immunostaining was found in subpopulations of bipolar and amacrine cells, but staining was weak in rod bipolar cells, and AII amacrine cells were immunonegative. Double labeling of sGC with neuronal nitric oxide synthase showed that the two proteins are generally located in adjacent puncta in inner plexiform layer, implying paracrine interactions. Our results suggest that the NO-cGMP pathway modulates the neural circuitry in inner retina, preferentially within the cone pathway.

Origin of electroretinogram amplitude growth during light adaptation in pigmented rats

We assessed the growth of the rat photopic electroretinogram (ERG) during light adaptation and the mechanisms underlying this process. Full field ERG responses were recorded from anesthetized adult Brown-Norway rats at each minute for 20 min of light adaptation (backgrounds: 1.8, 2.1, 2.4 log scotopic cd m(-2)). The rat photopic b-wave amplitude increased with duration of light adaptation and its width at 33% maximal amplitude narrowed (by approximately 40 ms). These effects peaked 12-15 min after background onset. The narrowing of the b-wave reflected steepening of the b-wave recovery phase, with little change in the rising phase. OP amplitudes grew in proportion to the b-wave. Inhibition of inner retinal responses using TTX resulted in a greater relative growth of b-wave and OP amplitude compared with fellow control eyes, and delayed the change in recovery phase by approximately 5 min. Inhibition of all ionotropic glutamate receptors with CNQX/D-AP7 delayed both rising and recovery phases equally (approximately 12 ms) without altering b-wave width or the time course of adaptation changes. These outcomes suggest that inner retinal light responses are not directly responsible for b-wave amplitude growth, but may contribute to the change in its recovery phase during adaptation. A TTX-sensitive mechanism may help to hasten this process. The cone a-wave was isolated using PDA/L-AP4 or CNQX/L-AP4. A-wave amplitude (35 ms after stimulus onset) also increased with time during light adaptation and reached a maximum (130 +/- 29% above baseline) 12-15 min after background onset. B-wave amplitude growth in fellow control eyes closely followed the course and relative magnitude of cone a-wave amplitude growth. Hence, the increase of the cone response during light adaptation is sufficient to explain b-wave amplitude growth.

Physiological properties of rod photoreceptor electrical coupling in the tiger salamander retina

Using dual whole-cell voltage and current clamp recording techniques, we investigated the gap junctional conductance and the coupling coefficient between neighbouring rods in live salamander retinal slices. The application of sinusoidal stimuli over a wide range of temporal frequencies allowed us to characterize the band-pass filtering properties of the rod network. We found that the electrical coupling of all neighbouring rods exhibited reciprocal and symmetrical conductivities. On average, the junctional conductance between paired rods was 500 pS and the coupling coefficient (the ratio of voltage responses of the follower cell to those of the driver cell), or K-value, was 0.07. Our experimental results also demonstrated that the rod network behaved like a band-pass filter with a peak frequency of about 2-5 Hz. However, the gap junctions between adjacent rods exhibited linearity and voltage independency within the physiological range of rods. These gap junctions did not contribute to the filtering mechanisms of the rod network. Combined with the computational modelling, our data suggest that the filtering of higher frequency rod signals by the network is largely mediated by the passive resistive and capacitive (RC) properties of rod plasma membranes. Furthermore, we found several attributes of rod electrical coupling resembling the physiological properties of gene-encoded Cx35/36 gap junctions examined in other in vitro studies. This indicates that the previously found Cx35/36 expression in the salamander rod network may be functionally involved in rod-rod electrical coupling.

Endothelium-dependent smooth muscle hyperpolarization: do gap junctions provide a unifying hypothesis?

An endothelium-derived hyperpolarizing factor (EDHF) that is distinct from nitric oxide (NO) and prostanoids has been widely hypothesized to hyperpolarize and relax vascular smooth muscle following stimulation of the endothelium by agonists. Candidates as diverse as K(+) ions, eicosanoids, hydrogen peroxide and C-type natriuretic peptide have been implicated as the putative mediator, but none has emerged as a 'universal EDHF'. An alternative explanation for the EDHF phenomenon is that direct intercellular communication via gap junctions allows passive spread of agonist-induced endothelial hyperpolarization through the vessel wall. In some arteries, eicosanoids and K(+) ions may themselves initiate a conducted endothelial hyperpolarization, thus suggesting that electrotonic signalling may represent a general mechanism through which the endothelium participates in the regulation of vascular tone.

The involvement of glutamate-gated channels in negative feedback from horizontal cells to cones

Photoreceptors are the light sensitive cells in the retina. They project to horizontal cells and bipolar cells via a glutamatergic feed forward pathway. Horizontal cells are strongly electrically coupled and integrate in that way the input from the photoreceptors. Horizontal cells feedback to cones negatively. The combined signal from the photoreceptors and the horizontal cells is sent to the bipolar cells. The feedback pathway from horizontal cells to cones is thought to form the basis for the center/surround organization of bipolar cells. The nature of the feedback pathway is an issue of intense debate. It was thought for a long time that this feedback pathway was GABAergic, because cones have GABA-receptors and horizontal cells release GABA via a GABA-transporter working in the reversed direction. However, recently we showed in goldfish that horizontal cells feed back to cones via an alternative mechanism. In goldfish, negative feedback from horizontal cells to cones shifts the calcium current of the cone to more negative potentials. This feedback pathway is independent of GABA, since feedback cannot be blocked by either saturating concentrations of PTX, the GABA-transporter blocker SKF89976A, or application of GABA. The mechanism of negative feedback from horizontal cells to cones involves hemichannels located at the tips of the invaginating horizontal cells, just opposite to the calcium channels of the cones. Current flowing through these hemichannels changes the extracellular potential deep in the synaptic cleft and in that way modulates the calcium current of the cones. Such a modulation of the extracellular potential is called ephaptic. If negative feedback from horizontal cells to cones is indeed ephaptic, other channels present in the synapse should also be able to act as a current source, i.e., should also be able to change the output of the cone. We showed that glutamate-gated channels present at the tips of the horizontal cell dendrites can also mediate feedback responses. Surprisingly, although the glutamate-gated conductance of the horizontal cells is eight times the hemichannel conductance, glutamate-gated channels are not the major current source in negative feedback from horizontal cells to cones. In this chapter we present evidence that this is due to the more focal localization of the hemichannels, compared to a diffuse and extrasynaptic localization of the glutamate-gated channels.

Inhibitory modulation of photoreceptor melatonin synthesis via a nitric oxide-mediated mechanism

Nitric oxide (NO) has been suggested to have many physiological functions in the vertebrate retina, including a role in light-adaptive processes. The aim of this study was to examine the influence of the NO-donor sodium nitroprusside (SNP) on the activity of arylalkylamine-N-acetyltransferase (AA-NAT; EC. 2.3.1.87), the activity of which responds to light and reflects the changes in retinal melatonin synthesis--a key feature of light-adaptive responses in photoreceptors. Incubation of dark-adapted and dark-maintained retinas with SNP lead to the NO-specific suppression of AA-NAT activity, with NO suppressing AA-NAT activity to a level similar to that seen in the presence of dopaminergic agonists or light. Increased levels of cGMP appeared to be causally involved in the suppression of AA-NAT activity by SNP, as non-hydrolysable analogues of cGMP and the cGMP-specific phosphodiesterase (PDE) inhibitor zaprinast also significantly suppressed AA-NAT activity, while an inhibitor of soluble guanylate cyclase blocked the effect of SNP. While this chain of events may not be part of the normal physiology of the retina, it could be important in pathological circumstances that are associated with marked increase in levels of cGMP, as is found to be the case in certain forms photoreceptor degeneration, which are produced by defects in cGMP phosphodiesterase activity.

Electrical coupling and neuronal synchronization in the Mammalian brain

Certain neurons in the mammalian brain have long been known to be joined by gap junctions, which are the most common type of electrical synapse. More recently, cloning of neuron-specific connexins, increased capability of visualizing cells within brain tissue, labeling of cell types by transgenic methods, and generation of connexin knockouts have spurred a rapid increase in our knowledge of the role of gap junctions in neural activity. This article reviews the many subtleties of transmission mediated by gap junctions and the mechanisms whereby these junctions contribute to synchronous firing.

Absence of circadian clock regulation of horizontal cell gap junctional coupling reveals two dopamine systems in the goldfish retina

In fish and other vertebrate retinas, although dopamine release is regulated by both light and an endogenous circadian (24-hour) clock, light increases dopamine release to a greater extent than the clock. The clock increases dopamine release during the subjective day so that D2-like receptors are activated. It is not known, however, whether the retinal clock also activates D1 receptors, which display a much lower sensitivity to dopamine in intact tissue. Because activation of the D1 receptors on fish cone horizontal (H1) cells uncouples the gap junctions between the cells, we studied whether the clock regulates the extent of biocytin tracer coupling in the goldfish retina. Tracer coupling between H1 cells was extensive under dark-adapted conditions (low scotopic range) and similar in the subjective day, subjective night, day, and night. An average of approximately 180 cells were coupled in each dark-adapted condition. However, bright light stimulation or application of the D1 agonist SKF38393 (10 microM) dramatically reduced H1 cell coupling. The D2 agonist quinpirole (1 microM) or application of the D1 antagonist SCH23390 (10 microM) and/or the D2 antagonist spiperone (10 microM) had no effect on H1 cell coupling in dark-adapted retinas. These observations demonstrate that H1 cell gap junctional coupling and thus D1 receptor activity are not affected by endogenous dopamine under dark-adapted conditions. The results suggest that two different dopamine systems are present in the goldfish retina. One system is controlled by an endogenous clock that activates low threshold D2-like receptors in the day, whereas the second system is controlled by light and involves activation of higher threshold D1 receptors.

High blood pressure and visual sensitivity

The study had two main purposes: (1) to determine whether the foveal visual sensitivities of people treated for high blood pressure (vascular hypertension) differ from the sensitivities of people who have not been diagnosed with high blood pressure and (2) to understand how visual adaptation is related to standard measures of systemic cardiovascular function. Two groups of middle-aged subjects--hypertensive and normotensive--were examined with a series of test/background stimulus combinations. All subjects met rigorous inclusion criteria for excellent ocular health. Although the visual sensitivities of the two subject groups overlapped extensively, the age-related rate of sensitivity loss was, for some measures, greater for the hypertensive subjects, possibly because of adaptation differences between the two groups. Overall, the degree of steady-state sensitivity loss resulting from an increase of background illuminance (for 580-nm backgrounds) was slightly less for the hypertensive subjects. Among normotensive subjects, the ability of a bright (3.8-log-td), long-wavelength (640-nm) adapting background to selectively suppress the flicker response of long-wavelength-sensitive (LWS) cones was related inversely to the ratio of mean arterial blood pressure to heart rate. The degree of selective suppression was also related to heart rate alone, and there was evidence that short-term changes of cardiovascular response were important. The results suggest that (1) vascular hypertension, or possibly its treatment, subtly affects visual function even in the absence of eye disease and (2) changes in blood flow affect retinal light-adaptation processes involved in the selective suppression of the flicker response from LWS cones caused by bright, long-wavelength backgrounds.